1.Field of the Invention
The present invention relates to the fabricating of an integrated circuit. More particularly, the invention provides an ArF photoresist resin and a preparation method therefor and, more particularly, to a photoresist copolymer suitable for submicrolithography using deep ultra violet (DUV) light as a light source and a method for preparing such a copolymer. Also, the present invention provides a photoresist composition (xe2x80x9cphotoresistxe2x80x9d) including such a resin.
2. Description of the Prior Art
Recently, chemical amplification photoresists have been prevailing in semiconductor devices since they have been found to be highly sensitive to DUV light, which is recognized as a light source suitable for accomplishing the high integration of semiconductor devices. A chemical amplification photoresist generally has a photoacid generator and a matrix polymer having a chemical structure which sensitively reacts with acid.
As for the reaction mechanism of such a photoresist, when the photoresist is exposed through a mask to a DUV light source, protons are generated by the action of the photoacid generator, which then reacts with the main or side chain of the matrix polymer. This reaction increases the solubility of the copolymer in a developing solution by converting the structure of the copolymer, e.g., by decomposing it, cross-linking it or changing its polarity. Therefore, when treated with the developing solution, the copolymer is dissolved at exposed regions and remains undissolved at unexposed regions, thereby leaving the shape of the mask as a positive image on a substrate.
Meanwhile the resolution of the patterns formed by photolithography is generally proportional to the wavelength of the light source. Thus, finer patterns can be formed as the wavelength is shorter. As a result of the effort to find new light sources suitable to improve the resolution, deep UV (DUV) light was developed for the integration of semiconductor devices into 1 Giga or higher scale.
Generally, photoresists are required to be of high etch resistance and thermal resistance. In addition, the photoresist to be used for an ArF (193 nm wavelength) light source should be developed in a 2.38% tetramethylammonium hydroxide (TMAH) solution. However, in fact, it is difficult to obtain a photoresist resin which satisfies those properties entirely.
For example, resins having a backbone of poly(methylmethacrylate), which is transparent to light of the above short wavelengths, are easy to synthesize. But there are problems in practical application owing to their poor etch resistance and development in TMAH solution. Etch resistance can be improved by introducing aliphatic ring monomers into the main chain. But it is virtually impossible to synthesize a resin having a main chain consisting of aliphatic rings.
In order to solve the problems, people such as those at ATandT (or Bell Laboratory) has developed a resin having a main chain which comprises norbornene, acrylate and maleic anhydride monomers, as represented by the following formula I: 
In Formula I, the maleic anhydride (part A) is used to polymerize aliphatic cyclo-olefin groups, but is dissolved in a 2.38% TMAH solution even in the state of unexposure. This dissolution can be inhibited by increasing the proportion of the y part of Formula I, the t-butyl substituent, in the main chain. If this is done, the z part, functioning to increase the adhesiveness to a substrate becomes relatively small in proportion, which leads to the release of the photoresist from the substrate, e.g. a silicon wafer. As a result, the formation of good patterns is impossible by this method. Bell Laboratory suggested a two-component system including a cholesterol compound as a dissolution inhibitor. This dissolution inhibitor is, however, required to be added in a large quantity, for example, about 30% by weight of the resin, so that Bell Laboratory""s resins are in principle problematic for use in a photoresist.
Therefore, it is an object of the present invention to overcome the above problems encountered in prior art and to provide an ArF photoresist resin which is dissolved only slightly in developing solutions without a chemical change in its structure in addition to being superior in etch resistance, thermal resistance and adhesiveness.
It is an object of the present invention to provide a photoresist copolymer.
It is another object of the present invention to provide a method for preparing the photoresist copolymer.
It is a further object of the present invention to provide a photoresist comprising the photoresist copolymer.
It is still another object of the present invention to provide a method for fabricating the photoresist.
It is still another object of the present invention to provide a method for fabricating an integrated circuit device.
It is still another object of the present invention to provide a partially complete semiconductor device.
The novel photoresist copolymers of the present invention comprise repeating units in the backbone of the polymer derived from one or more carboxy- substituted bicycloalkene monomers of the following Formula II: 
wherein, R represents hydrogen or a straight or branched alkyl group containing 1-10 substituted or non-substituted carbon atoms, R of one of said bicycloalkenes being hydroxyalkyl; and n is 1 or 2. As shown in Formula II, the monomers of the present invention are polymerized through the double bond in the bicycloalkene ring.
In Formula II, preferred R groups are hydrogen, 2-hydroxyethyl and t-butyl That is, preferred examples of the bicyclocalkene monomers include 2-hydroxethyl 5-norbornene-2-carboxylate, t-butyl 5-norbornene-2-carboxylate, 5-norbornene 2-carboxylic acid, 2-hydroxyethyl bicyclo [2,2,2] oct-5-ene-2-carboxylate, t-butyl bicyclo [2,2,2] oct-5-ene-2-carboxylate and/or bicyclo [2,2,2] oct-5-ene-2-carboxylic acid.
The copolymer of the invention has a molecular weight ranging from approximately 3,000 to 100,000.
Preferred polymers of Formula 11 comprise at least two carboxy-substituted bicycloalkene monomers and are represented by the following Formula II: 
wherein R1 is hydroxyalkyl; R2 is hydrogen or alkyl; n is 1 or 2; and x and y represent the relative amounts of each monomer. Preferably, for every monomer represented by x, there are two monomers represented by y, wherein R2 is hydrogen in one of said y monomers and R2 is t-butyl in the other y monomer.
Hydroxyalkyl groups represented by R1 in Formula IIA may have from 1 to 10 carbon atoms, preferably from 1 to 6 carbon atoms, and most preferably from 1 to 4 carbon atoms. Examples of preferred hydroxalkly groups are 2-hydroxyethyl, 2-hydroxy propyl, 3-hydroxy propyl, 2- hydroxy butyl, 3-hydroxy butyl, 4-hydroxy butyl and 2-methyl-3-hydroxy propyl.
Alkyl groups represented by R2 in Formula IIA are preferably blocking or protective groups that are clearable by acid generated by the photoacid generator used in the photoresist composition. A preferred alkyl group is t-butyl.
The copolymers of the present invention may be prepared by polymerization with only the alicyclic compounds represented by Formula II using known polymerization methods, for example using a metal catalyst system as described in Goodall et al, International Publication Number WO 96/37526. However, the preferred polymerization method for the practice of the present invention is to incorporate one or more additional monomers (hereinafter referred to as the polymerization-enhancing comonomers) to increase the yield of copolymer. The most preferred polymerization-enhancing comonomers are maleic anhydride, having the following Formula III: 
and/or vinylene carbonate of the following Formula IV: 
One of the preferred copolymers of the invention is prepared from vinylene carbonate and three bicycloalkenes wherein R in Formula II is hydrogen, 2-hydroxy ethyl and t-butyl, respectively, and n is 1. That is, in a preferred embodiment the three bicycloalkene monomers are 2-hydroxyethyl 5-norbornene-2-carboxylate, t-butyl 5-norbornene-2-carboxylate and 5-norbornene 2-carboxylic acid.
Preferred copolymers of the present invention may be prepared according to ordinary radical polymerization techniques using radical polymerization initiators in bulk polymerization or in solution polymerization processes. For the polymerization solvent, cyclohexanone, methylethylketone, benzene, toluene, dioxane, dimnethylformamide, and tetrahydrofuran alone, or combinations thereof, may be used. Usually, the polymerization is carried out in the presence of a polymerization initiator, such as benzoylperoxide, 2,2xe2x80x2-azobisisobutyronitrile (AIBN), actyl peroxide, lauryl peroxide and t-butylperacetate.
A positive photoresist composition useful for forming positive fine patterns in semiconductor devices may be obtained by mixing the novel photoresist copolymer of the invention with a photoacid generator in an organic solvent in a typical manner. The amount of the copolymer is dependent on the organic solvent, the photoacid generator and the lithography conditions, and is preferably about 10-30% by weight of the organic solvent used.
As an example of a method of fabricating a photoresist, the copolymer of the invention is first dissolved in cyclohexanone at an amount of 10-30% by weight and an onium salt or organic sulfonic acid, as a photoacid generator, is added at an amount of about 0.1-10% by weight of the photoresist polymer. Then, this solution is filtered with an ultra fine filter to yield a photoresist solution.
This photoresist solution is spin-coated on a silicon wafer and is, then, soft-baked at a temperature of 80-150xc2x0 C. for 1-5 min. in an oven or on a hot plate. An exposure process is carried out by use of a stepper which employs DUV light or excimer laser as a light source. Thereafter, the wafer is subjected to post-baking at a temperature of 100-200xc2x0 C. An ultrafine positive resist image can be obtained by immersing the post-baked wafer for 90 seconds in a 2.38% TMAH solution.
A better understanding of the present invention may be obtained in light of following examples which are set forth to illustrate, but are not to be construed to limit, the present invention.